This application is based on Japanese Patent Application No. 2001-139540 filed May 10, 2001, the contents of which are incorporated hereinto by reference.
1. Field of the Invention
The present invention relates in general to a method of producing a shoe which is formed of an aluminum alloy and which is disposed between a swash plate and a piston of a swash plate type compressor.
2. Discussion of the Related Art
A swash plate type compressor is adapted to compress a gas by converting a rotary movement of the swash plate into a reciprocating movement of a plurality of pistons. Between the swash plate which is rotated at a relatively high speed and each piston which is reciprocated at a relatively high speed, a shoe as a sliding member is disposed for permitting a smooth relative movement therebetween. In the swash plate type compressor which is required to have a reduced weight for use in an air conditioning system of an automotive vehicle, for instance, it has been proposed to use a shoe formed of an aluminum alloy.
The shoe formed of the aluminum alloy is produced, for instance, by a method comprising: a forging step of forging a blank into a shoe; a thermal refining step of thermally refining the shoe according to a T6 or a T7 treatment specified in the Japanese Industry Standard (JIS) H0001; and a grinding step of grinding the shoe to adjust its size, so that the shoe has desired dimensions. The shoe has sliding surfaces which are to be held in sliding contact with the swash plate and the piston, respectively. In operation, the shoe slides on both of the swash plate and the piston with lubricant oil films being formed between the sliding surfaces of the shoe and the sliding surfaces of the swash plate and the piston. Accordingly, suitable clearances need to be formed between the sliding surfaces of the shoe and the sliding surfaces of the swash plate and the piston. Therefore, the shoe is required to have a high degree of dimensional accuracy.
In the thermal refining treatment such as the T6 or T7 treatment, the shoe is subjected to heating, rapid cooling, etc., so that the shoe inevitably suffers from deformation due to the thermal refining treatment. Since some variation in the amount of deformation of the shoe due to the thermal refining treatment is inevitable, however, the forging operation in the forging step is arranged such that the shoe obtained after the forging step has a size which is larger than a nominal or desired value. In the grinding step following the thermal refining step, the grinding operation is effected on the shoe for adjusting its size to the nominal value. In the size-adjustment grinding operation, however, the required amount of stock removal is inevitably large, resulting in an increase of the time required for adjusting the size of the shoe after the thermal refining step and an increase of the cost of its manufacture.
It is therefore an object of the present invention to provide a method of quickly and economically producing a shoe formed of an aluminum alloy and having a high degree of dimensional accuracy. This object may be achieved according to any one of the following modes of the present invention, each of which is numbered like the appended claims and depends from the other mode or modes, where appropriate, to indicate and clarify possible combinations of elements or technical features of the present invention, for easier understanding of the invention. It is to be understood that the present invention is not limited to the technical features or any combinations thereof which will be described for illustrative purpose only. It is to be further understood that a plurality of elements or features included in any one of the following modes of the invention are not necessarily provided all together, and that the invention may be embodied without some of the elements or features described with respect to the same mode.
(1) A method of producing a shoe for a swash plate type compressor, the shoe being disposed between a swash plate and a piston of the swash plate type compressor and formed of an aluminum alloy, the method comprising: a main forging step of forging a blank for producing the shoe into a roughly-shaped precursor shoe; a thermal refining step of thermally refining the roughly-shaped precursor shoe; and a size-adjustment forging step of forging the roughly-shaped precursor shoe which has been thermally refined, into a size-adjusted shoe.
The method of producing a shoe according to the above mode (1) is characterized by conducting an additional or a supplemental forging operation to adjust the size of the roughly-shaped precursor shoe obtained after the main forging step and the thermal refining step. The shoe is inevitably deformed due to the thermal refining treatment in the thermal refining step, and the amount of deformation of the shoe due to the thermal refining treatment varies depending upon individual shoes. In the present method according to the above mode (1), the roughly-shaped precursor shoe which has been thermally refined is subjected to the additional forging operation for size-adjustment, for thereby permitting the obtained shoe to have a high degree of dimensional accuracy. Since the forging operation can be generally effected in a relatively short period of time, the size-adjustment forging step according to the above mode (1) of the present invention can be quickly effected without considerably increasing the time required for producing the shoe. Accordingly, the present method according to the above mode (1) permits an economical manufacture of the shoe.
The aluminum alloy used for producing the shoe is not particularly limited, but may be selected from among aluminum alloys conventionally used for producing the shoe and various known aluminum alloys. For example, it is possible to use an Alxe2x80x94Si alloy having a ratio of content of Al to Si which is nearly equal to that at which an eutectic mixture is formed. The Alxe2x80x94Si alloy is, for instance, A4032 specified in JIS H4100. The Alxe2x80x94Si alloy has a small coefficient of thermal expansion and exhibits a good wear resistance, so that the shoe formed of the Alxe2x80x94Si alloy exhibits good sliding characteristics. It is possible to use an Alxe2x80x94Cuxe2x80x94Mg alloy such as A2017, A2024 specified in JIS H4100, which alloy has a high degree of strength. The shoe formed of the Alxe2x80x94Cuxe2x80x94Mg alloy exhibits high degrees of strength and durability.
The shape of the blank used in the main forging step is not particularly limited, but may be suitably determined depending upon the shape of the shoe to be obtained. For instance, the blank may have a spherical shape, a hemi-spherical shape, a cylindrical shape, a disc-like shape, a truncated conic shape, or a truncated pyramid shape. Where the shoe has a part-spherical crown shape which will be described, it is desirable to use a cylindrical blank having a diameter which is smaller than an outer diameter of the shoe as the end product (hereinafter referred to as xe2x80x9cend product shoexe2x80x9d) and a height which is larger than that of the end product shoe, for permitting a forging operation at a relatively low forging ratio. The blank may be prepared according to any methods. For example, the blank is prepared by casting, punching of a plate member by a press, or cutting of a long cylindrical member. Where the blank has the cylindrical shape described above, the cylindrical blank is prepared first by extruding a billet having a predetermined shape and formed of an aluminum alloy which is obtained by casting and which has a predetermined composition, drawing the billet to provide a bar-shaped member having a predetermined diameter, and then cutting, by a shearing machine or a sawing machine, the bar-shaped member into pieces each having a predetermined length. The blank to be used in the main forging step is desirably subjected to an annealing treatment for facilitating the forging operation performed on the blank in the main forging step and obtaining a roughly-shaped precursor shoe which has good characteristics, e.g., a high degree of dimensional accuracy. The annealing condition varies depending upon the kind of the aluminum alloy of the blank. For annealing the blank, the blank is kept at a temperature of about 300-420xc2x0 C. for a predetermined time and then subjected to a slow-cooling such as an air-cooling or a furnace-cooling, for instance.
Each of the main forging step and the size-adjustment forging step may be effected in a hot or a cold state. Where the forging ratio is relatively high, the hot forging is preferably employed to prevent cracking on the surface of the shoe. Where the forging ratio is relatively low, the cold forging is preferably employed. The article obtained by the cold forging has a high degree of dimensional accuracy and a good surface condition. Further, the cold forging can be effected in a simplified and economical manner without heating. In the main forging step and the size-adjustment forging step, it is preferable to employ a closed-die forging operation which causes a plastic flow within the cavity of the die assembly, rather than a free forging operation, for permitting the forged article to have a high degree of accuracy in configuration and dimensions.
The thermal refining step is effected for the purpose of increasing a strength and a hardness of the shoe formed of the aluminum alloy, for instance. The thermal refining treatment conducted in the thermal refining step includes, for instance, a T4 treatment in which the roughly-shaped precursor shoe is subjected to natural aging by effecting a solution heat treatment, a T6 treatment in which the roughly-shaped precursor shoe is subjected to an artificial age hardening treatment after it has been subjected to the solution heat treatment, and a T7 treatment in which the roughly-shaped precursor shoe is subjected to a stabilizing treatment which will be described, after it has been subjected to the solution heat treatment. The strength and hardness of the shoe are considerably increased by the thermal refining treatment. The T4, T6, and T7 treatments are specified in JIS H0001.
The configuration of the size-adjusted shoe is not necessarily the same as that of the end product shoe which is installed on the swash plate type compressor. The size-adjusted shoe obtained after the size-adjusting forging step may be subjected to a surface treatment such as an electroless nickel plating. In this case, strictly speaking, the configuration of the size-adjusted shoe is not the same as that of the end product shoe which has been subjected to the surface treatment. Namely, the configuration of the size-adjusted shoe refers to a configuration of a base body formed of the aluminum alloy, which base body constitutes a substantial portion of the shoe and does not include a coating film or layer to be formed on its surface. The size-adjusted shoe may be subjected to a grinding operation for considerably small size-adjustment after the size-adjusting forging step. In this case, strictly speaking, the configuration of the base body of the size-adjusted shoe is not the same as that of the base body of the end product shoe which has been subjected to the size-adjusting grinding operation. The object of the method according to the present mode is to permit a quick size-adjustment operation by grinding, for instance, for thereby reducing the cost of the manufacturing of the shoe. As long as the object is attained, the slight difference between the configuration of the size-adjusted shoe obtained after the size-adjustment forging step and the configuration of the end product shoe to be installed on the compressor is not material in practicing the present method. The configuration of the size-adjusted shoe is considerably similar to that of the base body of the end product shoe installed on the compressor.
The roughly-shaped precursor shoe according to the above mode (1) has a higher similarity in configuration to the size-adjusted shoe than the blank. In the present method, the blank is forged, in the main forging step, into the roughly-shaped precursor shoe having a configuration which is very similar to that of the size-adjusted shoe, and the roughly-shaped precursor shoe obtained in the main forging step is subjected, after the thermal refining step, to the size-adjustment forging step wherein the forging ratio is lower than that in the main forging step. Namely, in the size-adjustment forging step, the rest of the forging operation is effected on the roughly-shaped precursor shoe to provide the size-adjusted shoe. Since the strength and the hardness of the shoe are increased after the shoe has been subjected to the thermal refining treatment, it will be difficult to effect, on the shoe, a forging operation in which the forging ratio is relatively high, where the cold forging is employed in the size-adjustment forging step. In view of this, the present method according to the above mode (1) is efficient. In the main forging step, a substantial part of the shoe is formed. The size-adjustment forging step can be referred to as xe2x80x9csizing forgingxe2x80x9d step.
The forging ratio of the roughly-shaped precursor shoe with respect to the size-adjusted shoe is defined as follows. Where the shoe is produced from the cylindrical blank by employing, in the main forging step and the size-adjustment forging step, the closed-die forging in the cold condition, the forging ratio is defined as a ratio of the height of the roughly-shaped precursor shoe with respect to the height of the size-adjusted shoe. Where the height of the size-adjusted shoe is represented by 100%, the forging percentage of the roughly-shaped precursor shoe (100xc3x97ratio of the height of the roughly-shaped precursor shoe with respect to the height of the size-adjusted shoe) is desirably held in a range of 101-110%. If the forging percentage of the roughly-shaped precursor shoe falls within the specified range, the roughly-shaped precursor shoe can be forged, in the size-adjustment forging step, into the size-adjusted shoe with a considerably high degree of dimensional accuracy. The forging percentage of the cylindrical blank (100xc3x97ratio of the height of the cylindrical blank with respect to the height of the size-adjusted shoe) is preferably held in a range of 105-140%.
(2) A method according to the above mode (1), wherein the main forging step comprises a plurality of sub-forging steps.
Where the blank needs to be forged to a great extent or the blank needs to be forged into a complicated shape by the cold forging, in particular, it may be difficult to cause an effective plastic flow of the material within the cavity of the die assembly, resulting in a deterioration in quality (e.g., dimensional accuracy) of the article to be obtained. In this case, if the forging operation on the blank is effected in a plurality of steps using different die assemblies, the degree to which the blank is forged in each step can be made low, so that the forged article has a high degree of dimensional accuracy and is free from any defects. Accordingly, the method according to the above mode (2) wherein the main forging step comprises a plurality of sub-forging steps permits an easy manufacture of the roughly-shaped precursor shoe having good characteristics, e.g., a high degree of dimensional accuracy.
(3) A method according to the above mode (2), further comprising an annealing step effected following at least one of the plurality of sub-forging steps except a last one of the plurality of sub-forging steps.
The forged article tends to suffer from work hardening (strain hardening) when the forging ratio is high, particularly where the cold forging is employed, making the subsequent forging operation difficult. In this case, the forged article is subjected to an annealing treatment, so that the article is softened, for thereby facilitating the subsequent forging operation. The annealing treatment in the annealing step effected between successive two forging steps is referred to as an intermediate annealing. The material of the article is softened by the intermediate annealing which is effective to promote the recovering process in which the lattice defect such as dislocation caused in the prior forging operation is eliminated or rectified. Accordingly, the present method according to the above mode (3) wherein the annealing step is effected following at least one of the plurality of sub-forging steps of the main forging step permits a forging operation at a relatively high forging ratio. The condition at which the annealing treatment is effected varies depending upon the kind of the aluminum alloy. When the annealing treatment is effected in a batch type furnace, the shoe is kept at about 300-430xc2x0 C. for about 2-4 hours. When the annealing treatment is effected in a continuous furnace, the shoe is kept at a temperature in the vicinity of 500xc2x0 C. for several tens of seconds, and then gradually cooled.
(4) A method according to the above modes (2), wherein an annealing step is not effected between any successive two of the plurality of sub-forging steps.
If the shoe is subjected to the annealing treatment described above, it requires a certain time period for annealing. Further, the annealing treatment requires an equipment such as a heating furnace and an energy source for heating the shoe, inevitably pushing up the cost of manufacture of the shoe. In view of this, the annealing treatment is not effected to reduce the cost of manufacture of the shoe as long as the forging operation can be performed effectively without the annealing operation. Therefore, the present method according to the mode (4) permits an economical manufacture of the shoe.
(5) A method according to the above modes (2), wherein the plurality of sub-forging steps consist of a first sub-forging step which is effected on the blank for obtaining an intermediate shoe whose similarity in configuration to the size-adjusted shoe is lower than the roughly-shaped precursor shoe, and a second sub-forging step which is effected on the intermediate shoe for obtaining the roughly-shaped precursor shoe.
In the method according to the mode (5), the main forging step consists of two sub-forging steps. The main forging step would be inevitably complicated if it included a large number of sub-forging steps. It is desirable to reduce the number of the sub-forging steps, where the forging ratio is relatively low in the main forging step or where the blank has a configuration which permits a relatively easy formation into the roughly-shaped precursor shoe. Accordingly, the present method according to the above mode (5) wherein the main forging step consists of two sub-forging steps permits a relatively simple and economical production of the shoe.
The intermediate shoe according to the above mode (5) has a lower similarity in configuration to the size-adjusted shoe than the roughly-shaped precursor shoe. The forging ratio of the intermediate shoe with respect to the size-adjusted shoe is defined in the same manner as described above with respect to the mode (1). In the above mode (1), the forging ratio of the roughly-shaped precursor shoe with respect to the size-adjusted shoe is represented by the ratio of the height of the roughly-shaped precursor shoe with respect to the height of the size-adjusted shoe, where the shoe is produced from the cylindrical blank by employing, in the main forging step (i.e., the two sub-forging steps) and the size-adjustment forging step, the closed-die forging in the cold condition. Where the height of the size-adjusted shoe is represented by 100%, the forging percentage of the roughly-shaped precursor shoe (100xc3x97ratio of the height of the roughly-shaped precursor shoe with respect to the height of the size-adjusted shoe) is preferably held in the range of about 101-110% as described above while the forging percentage of the cylindrical blank (100xc3x97ratio of the height of the cylindrical blank with respect to the height of the size-adjusted shoe) is preferably held in the range of about 105-140% as described above. The forging percentage of the intermediate shoe (100xc3x97ratio of the height of the intermediate shoe with respect to the height of the size-adjusted shoe) is preferably held in a range intermediate between the above-described two ranges, namely in a range of about 105-115%.
(6) A method according to the above mode (5), further comprising an annealing step effected between the first and the second sub-forging steps for annealing the intermediate shoe.
The present method according to the mode (6) enjoys advantages as described above with respect to the above mode (3). Accordingly, the present method according to the mode (6) wherein the annealing step is effected between the first and the second sub-forging steps of the main forging step permits a forging operation at a relatively high forging ratio. The annealing condition is similar to that described above with respect to the above mode (3).
(7) A method according to the above mode (5), wherein an annealing step is not effected between the first and the second sub-forging steps.
For the same reasons described above with respect to the above mode (4), the present method according to the mode (7) wherein the annealing step is not effected between the first and the second sub-forging steps permits an economical manufacture of the shoe.
(8) A method according to any one of the above modes (1)-(7), further comprising a size-adjustment grinding step effected on the size-adjusted shoe obtained after the size-adjustment forging step.
The method according to the present invention comprises the size-adjustment forging step for improving the dimensional accuracy of the shoe. The size-adjusted shoe suffers from a considerably small variation in the finished size due to the spring-back caused in the size-adjustment forging step. The present method according to the above mode (8) wherein the size-adjustment grinding step is effected on the size-adjusted shoe obtained after the size-adjustment forging step is effective to produce the shoe which is required to have a particularly high degree of dimensional accuracy.
(9) A method according to any one of the above modes (1)-(7), wherein a size-adjustment grinding step is not effected on the size-adjusted shoe obtained after the size-adjustment forging step.
By effecting the size-adjustment forging step, the size-adjusted shoe has a dimensional accuracy which is acceptable in view of the general requirements for the shoe. If the shoe is not required to have a particularly high degree of dimensional accuracy, the shoe can be produced effectively at a low cost by the present method according to the mode (9) wherein the size-adjustment grinding step is not effected on the size-adjusted shoe.
(10) A method according to any one of the above modes (1)-(9), further comprising a partial-forming step effected on a part of the blank prior to the main forging step.
If the blank has been subjected to the partial-forming step for forming a part of the shoe prior to the main forging step, which part generally has a complicated shape, the forging operations subsequently performed in the main forging step and the size-adjustment forging step can be easily effected, resulting in an improved dimensional accuracy of the obtained shoe. The above-indicated part formed prior to the main forging step is utilized as a reference for effecting various working operations in the subsequent steps such as the main forging step and the size-adjustment forging step, resulting in an improved dimensional accuracy of the obtained shoe. In view of this, the present method according to the mode (10) wherein the main forging step is effected on the partially formed blank is advantageous. The partial-forming step is effected according to any known methods such as machining, press working, and forging. For quickly effecting the partial-forming step, it is desirable to employ the press working or the forging.
(11) A method according to any one of the above modes (1)-(10), wherein the shoe for the swash plate type compressor includes a flat portion having a generally flat surface to be held in sliding contact with the swash plate, and a part-spherical portion having a generally part-spherical surface to be held in sliding contact with the piston.
The shape of the shoe produced according to the present invention is not particularly limited. Since the shoe is disposed between the swash plate and the piston in the swash plate type compressor, the shoe generally has a part-spherical crown shape described in the mode (11). It is particularly necessary to prevent the shoe from suffering from a variation in accuracy of configuration of the flat portion and the part-spherical portion each functioning as the sliding surface, or a variation in a positional relationship between the flat portion and the part-spherical portion, in other words, the height of the shoe. Accordingly, the present method which permits the production of the shoe having a high degree of dimensional accuracy is considerably advantageous for producing the part-spherical crown shoe.
The part-spherical crown shoe having a substantially spherical surface for engagement with the piston and a substantially flat surface for engagement with the swash plate is generally called as a hemispherical shoe. The flat surface may slightly deviate from a true flat surface while the spherical surface may slightly deviate from a true spherical surface for improving the sliding characteristics. Further, in general, the size of the shoe used for the compressor of variable capacity type is smaller than a hemi-sphere while the size of the shoe used for the compressor of fixed capacity type is larger than the hemi-sphere. Since the part-spherical surfaces of a pair of shoes which engage opposite surfaces of the swash plate of the compressor of the variable capacity type need to be located substantially on the same spherical surface, the size of each of the pair of shoes is made smaller by an amount corresponding to a half of the thickness of the swash plate. The size of the shoe used for the compressor of fixed capacity type is made slightly larger than the hemi-sphere for preventing a reduction in the sliding surface area even when the flat surface of the shoe is worn. The term xe2x80x9cpart-spherical crown shoexe2x80x9d is generic to the above-indicated two types of shoes.
(12) A method according to the above mode (11), wherein the shoe for the swash plate type compressor includes a flat potion having a generally annular flat surface which is formed with a recess at a substantially central portion thereof and which is to be held in sliding contact with the swash plate, and a part-spherical portion having a generally part-spherical surface to be held in sliding contact with the piston, the recess being formed in the partial-forming step.
The flat portion of the shoe which is held in sliding contact with the swash plate is subjected to a severe operating condition since the swash plate is rotated at a relatively high speed. In view of this, the flat portion of the shoe is tapered at its radially outer portion such that there is formed a clearance having a wedge-shaped cross sectional shape between the tapered portion of the flat portion and the swash plate when the shoe engages the swash plate. This tapered portion is effective to introduce a lubricant oil between the sliding surfaces of the shoe and the swash plate. For further improving the lubricating characteristic between the sliding surfaces of the shoe and the swash plate, the flat portion of the shoe is formed with a recess at a central portion thereof for accommodating the lubricant oil. When such a recess is formed in the main forging step, the plastic flow of the material is inhibited, making it difficult to form the flat portion having a desired configuration. If the recess is formed in the above-described partial-forming step effected prior to the main forging step, the flat portion having the desired configuration can be formed in the subsequent main forging step and the size-adjustment forging step. In forging the blank into the part-spherical crown shoe, it is desirable that the blank is located at a central portion of the cavity of the die assembly. If the recess formed at the central portion of the flat portion in the partial-forming step is utilized in positioning the blank relative to the die assembly such that the blank is located at the central portion of the cavity, a uniform or isotropic plastic flow of the material is caused in the cavity, permitting an improvement in the dimensional accuracy of the shoe to be obtained. The present method according to the above mode (12) relating to the part-spherical crown shoe having the recess formed at the central portion of the flat portion enjoys the advantages described above.
(13) A method according to any one of the above modes (1)-(12), wherein each of the main forging step and the size-adjustment forging step is effected by cold forging.
As explained above, the cold forging permits the obtained article to have a high degree of dimensional accuracy and a good surface condition. Further, the cold forging can be performed in a simplified and economical manner without heating. Accordingly, the present method according to the above mode (13) wherein each of the main forging step and the size-adjustment forging step is effected by cold forging enjoys the advantages described above.
(14) A method according to any one of the above modes (1)-(13), wherein the thermal refining step effected on the roughly-shaped precursor shoe comprises: a step of effecting a solution heat treatment; and a step of effecting an artificial age hardening treatment after the step of effecting the solution heat treatment.
The thermal refining treatment performed in the thermal refining step according to the above mode (14) corresponds to a T6 treatment specified in JIS H0001. The T6 treatment is performed to permit the article to exhibit the maximum strength and hardness. In the present method according to the above mode (14) wherein the T6 treatment is employed in the thermal refining step, the shoe to be produced has considerably high degrees of strength and hardness. In the solution heat treatment according to the above mode (14), the shoe is kept in a heating furnace at about 490xc2x0 C. for a time period ranging from 0.5 hour to 6 hours, and then rapidly cooled to room temperature. In the artificial age hardening treatment, the shoe is kept in the heating furnace at about 180xc2x0 C. for 2-6 hours.
(15) A method according to any one of the above modes (1)-(13), wherein the thermal refining step effected on the roughly-shaped precursor shoe comprises: a step of effecting a solution heat treatment; and a step of effecting an over-aging treatment which is effected beyond conditions of an artificial age hardening treatment at which the maximum strength is obtained and which is effected after the step of effecting the solution heat treatment.
The thermal refining treatment performed in the thermal refining step according to the above mode (15) corresponds to a T7 treatment specified in JIS H0001. The dimensional stability of the shoe which has been subjected to the T7 treatment is improved though the strength and hardness of the shoe are slightly lowered. Therefore, the present method according to the above mode (15) permits the production of the shoe which exhibits a high degree of stable dimensional accuracy. In the solution heat treatment according to the above mode (15), the shoe is kept in the heating furnace at about 490xc2x0 C. for a time period ranging from 0.5 hour to 6 hours, and then rapidly cooled to room temperature. In the over-aging treatment, the shoe is kept in the heating furnace at about 200xc2x0 C. for 3-6 hours. The over-aging treatment according to the mode (15) is also referred to as xe2x80x9ca stabilizing treatmentxe2x80x9d. In this specification, the term xe2x80x9cstabilizing treatmentxe2x80x9d is used.